Telescopes used in many industries comprise large, sophisticated computer-controlled instruments with full digital outputs. And whereas telescopes have evolved over time, designers have paid particular attention to telescope parameters, including the light-collecting power of the telescope (as a function of the diameter of the telescope) and the angular resolution (as measured by image sharpness). For a perfect telescope operated in a vacuum, resolution is directly proportional to the inverse of the telescope diameter. In this regard, the perfect telescope generally converts a plane wavefront from distant star (effectively at infinity) into a perfectly spherical wavefront, thus forming the image with an angular resolution only limited by light diffraction.
In practice, however, errors such as atmospheric and telescope errors distort the spherical wavefront, creating phase errors in the image-forming ray paths. Generally, the cause of such atmospheric distortion is random spatial and temporal wavefront perturbations induced by turbulence in various layers of the atmosphere. Image quality can also be affected by permanent manufacturing errors and by long time scale-wavefront aberrations introduced by mechanical, thermal, and optical effects in the telescope, such as defocusing, decentering, or mirror deformations generated by their supporting devices.
In light of the errors introduced into such telescope systems, mechanical improvements have been made to minimize telescope errors. As a result of requirements for many large telescopes, typically those with primary mirrors above one meter, a technique known as adaptive optics was developed for medium or large telescopes, with image quality optimized automatically by means of constant adjustments by in-built corrective optical elements. In this regard, telescope systems operating according to the adaptive optics technique generally include an adaptive optics assembly that comprises a deformable mirror that is optically coupled to the telescope behind the focus of the telescope at or near an image of the pupil. The deformable mirror, which includes a number of actuators for essentially changing the shape of the mirror, is controlled to apply wavefront correction to images received by the telescope.
In addition to the adaptive optics assembly, such telescope systems also generally include a tracking system. Whereas such conventional tracking systems are adequate in tracking objects imaged by the telescope system, such tracking systems have drawbacks. As will be appreciated, the effectiveness of the closed-loop control of the tracking system in tracking the movement of the object is generally limited by the rate at which a tracking device, such as a tracking charge-coupled device (CCD) focal plane, can record an image received from the telescope system.
Because of the limit of the imaging device, some movement of the object, or residual jitter, of the object between each image taken by the focal plane array can escape the tracking system and cause degradation of images taken by the adaptive optics assembly. And as objects being tracked emit or reflect a decreasing amount of light, thus reducing the intensity of light received from the object, the ability of the tracking system to compensate for residual jitter decreases. In this regard, the time required for the tracking device to collect enough photons of light to exceed the dark-cell current of the focal plane to thereby adequately image the object increases as the object becomes dimmer. And as the time required for the tracking device to image the object increases, the ability of the tracking system to compensate for residual jitter decreases.